This invention relates generally to radio frequency (RF) plasma spray deposition devices and particularly to apparatus and methods for deposition at frequency levels of less than about 1 MHz.
Radio frequency (RF) plasma deposition is a plasma spray process which is well known for producing high temperature gaseous plasma. The devices for generating the plasma are sometimes referred to as plasma guns. They find utility in diverse heating applications such as high temperature chemical reactions, heating of solid targets, melting of particles such as a superalloy and for providing surface coatings and spray processes. Plasma processes are also used to produce low interstitial content titanium, refractory metal, as well as the superalloy deposits. In addition, the deposition efficiency of materials sprayed by the RF plasma process can approach 100%.
RF plasma deposition is a plasma spray process which can be used to fabricate low interstitial content titanium, refractory metal, and superalloy deposits. For example, U.S. Pat. No. 4,805,833, the disclosure of which is incorporated herein by reference, describes an RF plasma apparatus, including an RF plasma gun and the operation thereof in a frequency range of from 2 to 5 megahertz. The plasma is produced by induced RF energy which causes gases flowing in the interior of the gun to form a plasma plume or jet which flows to the adjacent substrate.
Efforts to develop techniques for operating RF plasma devices at lower frequency levels were undertaken. It was found that operation of the guns at frequencies less than about 1 MH reduced the ability of the gun to adequately heat a full range of alloys and particle sizes. At low frequency levels, the plasma guns experience difficulty in power coupling to the plasma. In addition, conventional gas mixtures and gun designs which operate well at 2 MHz tend to degrade or crack the quartz tube portion of the gun which encloses the plasma when operated at frequency levels of about 400 KHz.
Accordingly, a need exists for the successful deposition of feed material using RF plasma spray guns with improved particle heating and without the disadvantages experienced with the use of known RF plasma deposition techniques and conditions.
It is an object of this invention to provide a plasma gun operable at low RF frequencies.